This invention relates to the production of pigment dispersion starting with a tank flushed pigment using the twin screw. This approach gives a lower conductivity and better dispersed material without affecting the throughput of the process.
Conventional flush colors are made by using a Sigma blade mixer where the pigment presscake and the oleoresinous vehicle are mixed in the presence or absence of alkyds in portions to get a stiff paste. The water which breaks out during the process is decanted. The final stiff paste (flush) is dried under vacuum to eliminate the residual water. The dried product is then cut back with solvents, oil and the vehicle to make a standardized flush. This process is time consuming and requires heavy duty machines to accomplish the task. Further, it is a batch process and variation due to presscake quality, solids content of pigment and vehicle characteristics will result in wide variation in quality.
U.S. Pat. No. 4,474,473 illustrates a process to make pigment dispersions continuously using a twin screw co-rotating extruder wherein the hydrophobic liquid organic liquid media and the wet pigment presscake are fed into the extruder, and the pigment presscake is flushed and dehydrated continuously to obtain the pigment dispersion. The drawback of this process is the pigment presscakes used should be very high in solids content to achieve flushing. Further, the organic liquid media and the wet cake must be metered at proper rates to achieve flushing and dehydration. The other problem is that if the presscake is not washed well, there is no provision for washing in the process to reduce the conductivity of the resulting flush.
The present invention provides a process for the preparation of a pigment dispersion, said method comprising:
is (a) providing a twin screw extruder which comprises a pair of co-rotating parallel screws, a feed section, a dispersion section and a dehydrating section;
(b) feeding a tank flushed pigment into said feed section of the twin screw extruder and feeding a hydrophobic liquid organic media into the twin screw extruder;
(c) continuously flushing pigment particles from a water phase to an organic medium phase, thus forming a flushed pigment mixture; and
(d) transferring said flushed pigment mixture from said flushing section to said dehydrating section and therein continuously dehydrating said mixture by rotation of said pair of screws thereby forming pigment dispersion,
wherein feeding the hydrophobic liquid organic media is done at any time prior to or after the dehydration of step.
It has now surprisingly been found that using a tank flushed pigment (preflushed pigment) slurry or presscake when fed into a co-rotating extruder equipped with a feeding section, and multiple washing sections to achieve proper pH and conductivity levels, and a dehydrating section to remove the water, provides a method to make a consistent quality flush. The extruder can be equipped with sections to charge components to add prior to dehydration or after dehydration to get the desired rheology of the resulting flush.
The advantage of the process is that it can get a good dispersion of the pigment using the preflushed pigment or presscake. The rheology of the flush can be controlled by changing the process parameters.
Preferably, the twin screw extruder used in the present invention consists of 15 barrels (sections) with a feed section, 3 wash zones, 3 vent or vacuum zones and 1 discharge. The barrel sections are provided with water cooling and electric heating. The maximum screw speed of the extruder is 588 rpm and maximum power is 40 kW. The feeding of the preflushed material is preferably done using a K-Tron S60 feeder.
For the lithol rubine (C.I. Red 57) two kinds of preflush material were processed through the twin screw, the unwashed and washed preflush. For the unwashed material, the necessary washing was accomplished in the wash zones to reduce conductivity.